The contribution of Yu.N.Denisyuk, E.Leuth, and Yu.Upatnieks into holographic science and technology is
considered from the point of view of the modern state in holography. Some biographical data of these outstanding
specialists are given, and their significance in the holography development is estimated.
KEYWORDS: Holograms, Holography, Information theory, Diffraction, 3D image reconstruction, Spatial frequencies, Wavefronts, Telecommunications, Data storage, Signal attenuation
Two important features make a holographic system more substantial than an ordinary optical system: ability of phase
information transmission (on a level with amplitude one) and ability of the separable overlapping of wavefronts from
different objects. However, holographic signal is more complicated than that transmitted by ordinary optical devices.
Due to this the statements of information theory require some modifications in a higher degree for holographic systems
than for optical ones in order to provide the information transmission description.
In connection with 60th anniversary of new optical division - holography - the significant changes are considered which
occurred due to holography in the area of recording and storage of information carrying by light. New recording media
are discovered, and some old recording media are significantly improved, new recording methods of light information
recording and storage are proposed. It is noted that despite the prominent achievements, a very wide work regarding
holographic information recording and storage media and methods lies ahead in order to realize the possibilities of
holography more completely.
KEYWORDS: Spatial frequencies, Chemical elements, Spectral resolution, Signal to noise ratio, Televisions, Diffraction, Information theory, Polarization, Holograms, Particles
Light information devices were created a long time before the information theory was developed, and the features of these devices were described by the common optical characteristics. However, these characteristics look insufficient for these devices information abilities definition, and the need for specific information characteristics has become apparant. Along with that, the common methods for optical characteristics estimation must be applied for the definition of information characteristics. It has been necessary to compare some optical characteristics (input signal value expressed in amplitude and phase, resolving power, signal distribution by light wavelengths or color components, by polarization plane position, and by time intervals) with the information ones. In order to come to information characteristics, it was necessary to perform optical signal and object (and image) light field digitization. In this work we discuss and compare different methdos of the signal and image light field division into different componenets (freedom degrees): determined, statistical, and system-based.
KEYWORDS: Interference (communication), Signal to noise ratio, Spatial frequencies, Information theory, Signal attenuation, Wavefronts, Modulation transfer functions, Optoelectronic devices, Spectral resolution, Information technology
Fast development of information technologies results the need to use information characteristics in order to describe the
optical and optoelectronic devices. The statements formulated in the information theory for the communication systems
can be also used in information optics, if the optical field digitization problem is solved. The problems which proceed
from different possibilities of light signal and field digitization, are considered below.
The difference between conditions of image and hologram recording is determined by the fact that in the first case the
light intensity spatial distribution whereas in the second case the light wave amplitude and phase distribution is recorded.
Also multiplexing is possible for the hologram recording, which increases significantly the informative capabilities of
the recording. The phase recording possibility allows to create several new classes of recording media. In order to
improve the media successfully, it is necessary to understand correctly the demands to them which are discussed below.
The features of the media acceptable for hologram recording as well as their difference as concerns the recording
mechanism and characteristics are considered. Special attention is paid to the performance of the recording processes in
time domain. The variations occurring during the hologram storage and image reconstruction from the recorded
holograms are also the objects of our attention. The differences in the hologram recording mechanisms and kinetics
allow holographic solving of different problems.
The comparative analysis of the characteristics for combined correlators-interferometers is given for the cases of two-beam recording in thin and thick media. It has been noted that the thick medium recording provides the output data bigger accuracy than thin medium recording. Another advance of thick medium recording is the possibility to obtain the set of correlograms describing the object state variations in different moments. This possibility appears due to the multiplexing procedure performance.
KEYWORDS: Holograms, Data storage, Holography, Diffraction, Multiplexing, Spatial light modulators, Volume holography, Photodetectors, Signal to noise ratio, 3D image reconstruction
The times of information input and access during the processes of a hologram recording and reconstruction basically depend on the amount of information stored in a medium. This dependence is different for different media. Besides dependence on each other, these values depend on the medium characteristics such as sensitivity dynamic range (DR) and medium indices defining the hologram diffraction efficiency (DE). Amount of the stored information, access and input times - important parameters for holographic memory devices (HMD) -- are of different importance for different problems solving. Usually the total information amount and data access rate are of the biggest importance. In this case it is necessary to provide wide DR of a medium, multiplexing possibility, and recording of phase holograms which have bigger DE. For large amount of information fast input it is necessary to provide high holographic sensitivity. Its threshold value is defined by the value reciprocal to input energy enough to produce DE providing the information unit reconstruction. For DE providing transmission of N information units the energy must be increased in more than N times. Such recording media as lithium niobate and some photopolymers have the characteristics which allow to provide very high information capacity, acceptable access rate but insufficient holographic sensitivity which allows to input large data amount only at expense of parallelism. The medium characteristics variations allow to provide the optimal relationship of the parameters.
Information input, storage, and sampling using the holographic methods can be performed as a result of using some processes part of which are thermal, and some of the others depend on the temperature modes. Depending on the way how do these processes go, the most important characteristics of the holographic systems (such as the stored information density and amount, information transmission rate, and information quality) can vary. The behavior of the listed characteristics on the temperature modes of information recording, storage, and sampling has been considered in the presented paper qualitatively.
The estimation of information quality -- value which takes into consideration both system information capacity and rate of data arbitrary access -- has been performed for the volume holographic memories. It has been shown that if the technical possibilities are limited, the super high information capacity of the volume holographic memories exceeds those of the other kinds of memory.
KEYWORDS: Holograms, Diffraction, Multiplexing, Holography, Data storage, Signal to noise ratio, Volume holography, 3D image reconstruction, Diffraction gratings, Refractive index
The possibility to use the third dimension of the medium for data storage and extraction in memory devices is accessible in a wide sense only if a holographic method of data recording and reconstruction is used. However, this possibility has many limitations part of which is inherent just to the holographic devices. Among them one can find significant influence of a limited dynamic range, quadratic dependence of power expenses on the amount of stored information, limitations of the number of selective positions which can be used for the hologram multiplexing as well as some geometric limitations which are significant in 3-D holographic memory. On a level with that, such phenomena exercise influence on holographic memory device information capability as diffraction limits of information input and storage, spatial information losses in a complex system, limitations of rate of information input and output in holographic memory devices etc. The limitations caused by each of the listed factors have been compared and analyzed. It has been found that some of these factors do not influence on information capability limitations provided by the other reasons.
KEYWORDS: Holograms, Head-mounted displays, Spatial light modulators, Diffraction, Holography, Volume holography, Lithium, Multiplexing, Data storage, Signal to noise ratio
The role and place of the spatial light modulators (SLM) in holographic memory devices (HMD) with by-page data storage have been estimated. The conditions under which the SLM characteristics are optimized for the information input into HMD, have been considered. It has been shown that it is not enough to make the demands to the SLM harder but it is necessary that they would not be set too high.
The peculiarities of wavefront reconstruction under the condition of simultaneous illumination of the volume holograms by the object and reference beams, have been considered when the reconstructing object beam partially differs from the recording object beam by its phase distribution. A real time interferometry and correlation analysis of initial and final states of the object can be carried out simultaneously under the combined illumination both for volume and plate holograms. However, the case of volume holograms is characterized by bigger efficiency which is caused by the difference of diffraction at volume and plane structures.
KEYWORDS: Spectral resolution, Signal to noise ratio, Optical signal processing, Televisions, Acousto-optics, Data processing, Holography, Diffraction, Signal processing, Array processing
The mutual influence of the spatial and temporal parameters variations has been considered for the optical information processing systems. These parameters general dependence on the system physical units has also been considered. The possibilities to optimize the systems due to the more important parameter improvement at expense of less important one (spatial parameter at expense of temporal one or vice versa) have been estimated, and the possibilities to improve both kinds of the parameters simultaneously have been searched.
In order to describe acousto-optical spectrum analyzers (AOSA) it is necessary to take into consideration not only spatial resolving power defining the amount of the spectrum intervals to be distinguished (which is usually considered) but also data temporal resolving power for temporally variable spectra. These parameters are intercepted, and changing of one influences both on variation of the others and on the AOSA total information efficiency (amount of useful information extracted by the device from the variable signal of certain duration). It has been shown that increasing of the signal transmission through the acousto- optic Bragg cell exercises influence on the AOSA information efficiency by the complicated way. The bigger this time, the bigger amount of the spectrum resolved intervals (which means that the information capacity becomes bigger), the less the signal useful duration, and the less the AOSA information transmission capability for the devices processing the signal with periodically variable spectrum. Along with that part of the time is spent for mixed distorted signal. In order to optimize AOSA from the point of view of useful information transmission, it has been proposed to assign different weight factors to each of kinds of information, and, correspondingly, to each of the device parameters. These weight factors define valuability of certain kind of information. Their using for the quantitative estimation of the useful information amount, allows to create the basis of AOSA information metrology.
Acousto-optic deflectors play an important part in holographic memory devices (HMD). However, these deflectors not only expand the HMD possibilities but also introduce some limitations related to the maximum amount of information to be stored as well as to the rate of information retrieval. These limitations are the subjects of the consideration in the presented paper.
Acousto-optic deflectors (AODs) are used for data by-page recording as holograms, and for data extraction from the hologram sampled by the AOD. The possibilities and characteristics of AODs define information capacity of the holographic memory devices as well as architecture and parameters of their separate components. Usually 2 AODs are used for 2-D hologram recording, and 3 AODs -- for volume multiplex recording. The calculations of amount of the recorded holograms which can be provided by different AODs have been carried out in the present work, and the limits of information capacity increasing at expense of optimal using of both AODs and recording media, have been appointed. The specific attention have been paid to the hologram 3-D recording with angular multiplexing where information capacity can be increased by 2 orders. The difficulties to be overcome in order to achieve the biggest information capacity, have been noted. Another subject of study is the problem of the sampled data arbitrary access rate increasing which can be provided by AODs application. It has been noted that minimum access time is limited, and this limitation is connected with the duration of the signal transmission through Bragg cell and with the time of signal transmission and processing in other components of holographic memory. It has been shown that information quality of holographic memory grows along with increasing of the number of used AODs. It has also been noted that if the memory parameters are close to their limits, the cross-talks grow, and the optimum must be found.
A correlator-interferometer for the phase objects real time recognition has been developed. The certain conditions under which it may operate successfully, have been examined.
In order to calculate information transmission through an acousto-optical device and find the places of main information losses it is necessary to follow the way in which the signal transmits through such a device. The signal transmission is described by means of consideration of signal transmission function. The way of the signal is divided into several chains each of which performs the certain conversion of the signal corresponding to the certain physical process. The transmission functions of separate chains are calculated from the point of view of description of information transmission through an acousto-optical device.
The reasons of the limitations of the total information capacity of holographic memory devices (HMD) with the short retrieval duration and two stages of infirmation recording and sampling, have been considered. The theoretical and practical optimum is found for the possible variations of number of the subholograms and number of pixels in a hologram. The possibilities of HMD information capacity increasing without the significant raising of the retrieval duration have been noted.
The optimization of the ratios of recording and reconstructing object and reference beams, and also the effect of the dynamic range, resolution and hologram size on the informativity of interferograms are considered, which results in obtaining the higher informativity of real-time holographic interferograms.
Mutual influence on the holographic memory devices (HMD) characteristics has been considered. The possibility of their variation by the way of the controlled change of one of these characteristics, and by the way of HMD separate links parameters optimization and matching has been shown. The special attention has been payed to the optimization of parameters for such important link as acousto-optical device.
The possibilities and conditions of high spatial frequencies transition are evaluated in the present work for the photothermic spatial light modulators (SLMs) in which the recording is carried out to the peritectic semiconductor compositions such as thallium telluride under the action of laser irradiation short pulses. Such photothermic SLMs may store recording at room temperature which is a distinguishing peculiarity this SLM among the others realizing recording due to the phase transition of the second of the second kind. It shown that high resolving power is achieved with short pulses, and optical recording is performed both at expense of the absorption variation and at expense of variation of the recording medium films polarization properties.
It is shown that if a hologram is illuminated simultaneously by not only the reference but also object beam, so the wavefronts patterns provide additional information on an object, for example, reflect the correlation between initial and current states of phase object.
Holographic memory devices (HMD) are considered from the point of view of the possibility to minimize the light power spent for unit of the recorded and sampled information. The range of this power value is evaluated. The difference in specific power expenses for different HMD versions is noticed and the conditions under which these expenses may be minimal are analyzed.
The peculiarities of the wavefronts formation during the phase objects hologram reconstruction simultaneously by the reference and object beams are discussed. The interference patterns which appear if the reconstructed waves and those passed through the hologram without deflection are superposed, carry information on the reconstructed beams variations. Influence of the factors describing the considered system on the correspondence of the interference patterns to the variations of the reconstructed beams is evaluated.
Light power spent per information unit in the holographic memory is evaluated. Connections between the separate components of this power are established. It is shown that quantum efficiency of information which is sampled consecutively appears to be small if the retrieval rate of the separate bit of information is high. The value of the information unit quantum efficiency grows sufficiently if the retrieval of the word or image is parallel.
The reasons for the spatial information filling factor limitations in holographic memory devices (HMD) are analyzed and the significance of a recording medium dynamic range among those factors is estimated. Connection between the information filling factor and dynamic range are investigated for different recording media.
KEYWORDS: Holograms, Signal to noise ratio, Head-mounted displays, Data storage, Holography, Analog electronics, Digital recording, Fourier transforms, Spatial light modulators, Reliability
An important parameter of holographic memory devices (the filling factor of data accessible for sampling without losses) is considered. The reasons this factor is usually very small in different systems are analyzed.
Spatial and temporal processes going when optical parameters distribution is formed under the influence of spatially modulated light pulses in photothermic spatial light modulators (SLM) are studied. Possibilities of spatial information transmission through photothermic media with spontaneous erasing and erasing caused by additional pulses are evaluated on the basis of analysis being performed. The possibility to obtain sufficient diffraction efficiency for spatial frequencies about 1000 lines/mm is shown.
The criterion combining the information capacity and the average time of selection is suggested for potential possibilities of various memory devices to be compared with each other. It has been shown that according to this criterion a HMD has some advantages compared to the other memory devices due to two-step addressing. The optimization of HMD because of the right selection of hologram number correlation in a memory device has been noted to be possible.
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